1. Field of the Invention
This invention relates to photographic base paper (often abbreviated to "photobase") and to methods of making such paper.
2. Background Art
Photographic base paper comprises a fibrous substrate which may be wholly cellulosic or may include a proportion of synthetic polymeric fibres. Typically, the base paper has a weight of around 150 g/m.sup.2. To prepare the paper substrate for reception of a layer of photosensitive material e.g. photographic emulsion, the substrate is provided with a coating (usually a polyolefin or barium sulphate) on to which the emulsion is then itself coated. If a high gloss on the surface of the photographic emulsion is required, the surface of the emulsion is required to be extremely smooth.
The smoothness of the surface of the emulsion coating is largely determined by the smoothness of the surface of the underlying paper substrate, so the achievement of a smooth substrate is of critical importance. The requirement for smoothness is so high that most methods of measuring surface smoothness are not sufficiently sensitive to assess the suitability of a particular paper for making photographic prints and can only be used as an approximate guide. Visual assessment by skilled assessors is normally relied upon.
Typically, the photo-base paper-making "furnish" (that is, the pulp supplied to the headbox of the Fourdrinier paper-making wire) contains bleached softwood sulphite pulp and bleached hard-wood sulphite pulp, for example two parts of soft-wood to one of hard-wood, refined to a degree of wetness of twenty-five to thirty-five degrees on the Schopper-Riegler scale (25-35.degree. SR). The paper is usually wet-strengthened with aminoplast resins and sized with natural or synthetic sizes and with starch. It may be calendered to a density of from 1.0 to 1.25.
Special pulps are chosen in order to achieve sufficient smoothness. The pulps in question are known as the "photographic" grades and are characterised by very high whiteness and brightness, the absence of dirt and chemical impurities, and by the content of soft and conformable fibres. Typically, such pulps have a high content of alpha cellulose and are made from soft woods by a slow and expensive digestion process. They are consequently expensive. Available sources for such pulps are well-known in the industry.
Some rigidity is highly desirable in photographic papers, but the soft and conformable nature of the component fibres, which provides the necessary surface smoothness, tends to limit the rigidity of the paper. A relatively rigid paper can better withstand the stresses which arise from the expansions and contractions of the photographic emulsion during its drying, and during frequent handling, and resist curling resulting from changes in ambient conditions.
An improvement in rigidity can be achieved, at the expense of smoothness, by modifying the kinds and/or treatment of the pulp used. Increasing the rigidity by increasing the thickness of the sheet is unacceptable since it renders the product photobase, and photosensitive papers made from it, difficult to handle, for example, in reels and cassettes. For wide acceptability, the weight of the photobase paper should lie within a range of from 140 to 165 gm.sup.-2.
Thus, the simultaneous demands for extreme surface smoothness, and for rigidity, are mutually contradictory.